A needle assembly includes a tube holder for receiving an evacuated fluid collection tube. A needle cannula projects distally from said tube holder and a tip guard is slidably movable along said needle cannula from a proximal position near said tube holder to a distal position for shielding the distal end of the cannula. An actuator arm extends proximally from said tip guard and releasably engages the holder when the tip guard is in the proximal position. Insertion of a tube into the tube holder releases the actuator arm and enables the tip guard to be propelled to the distal position.
